1. Field of the Invention
The present invention relates to programming content addressable memory utilizing phase change memory and an access transistor.
2. Description of Background
Content addressable memory (CAM) is a type of computer memory utilized in high speed searching applications. Most CAM devices utilize static random access memory (SRAM) as data storage devices (utilizing transistors to store memory), and additional transistors and complementary transistors for match operations. Often in these CAM devices search-line access elements and word-line access elements are necessary to operate and program individual memory cells in the memory arrays. These search-line access elements and word-line access elements are often comprised of power intensive large drive field effect transistors (FET).
Phase change material can also be utilized to store memory for CAM devices. Information is stored in materials that can be manipulated into different phases. Each of these phases exhibits different electrical properties that can be used for storing information. The varying ratios of the amorphous and crystalline phases have detectable differences in electrical resistance. Specifically, the greater the ratio of amorphous phase, the higher the resistance.
Chalcogenides are a group of materials commonly utilized as phase change material. They typically contain a chalcogen (Periodic Table Group 16/VIA) and a more electropositive element. Selenium (Se) and tellurium (Te) are the two most common semiconductors in the group used to produce a chalcogenide when creating a phase change memory cell. An example of this is Ge2Sb2Te5 (GST), SbTe, and In2Se3. However, some phase change materials do not utilize chalcogen, such as GeSb. Thus, a variety of materials can be used in a phase change material cell.
Two problems in phase change CAM devices and standard CAM devices are the size of individual memory cells in the memory devices (requiring complementary access transistors and complementary memory elements) and the amount of information a phase change memory cell can store. Thus, it is desirable to devise a CAM configuration to reduce the size of the individual memory cells and to be able to store a greater amount of information.